The scaling of advanced devices such as integrated circuit based semiconductor devices to ever smaller dimensions relies heavily upon continued improvement in device materials, lithography and etching to process a device during fabrication. As the critical dimension (CD) of integrated circuit devices such as logic chips, memory chips, and system-on-a chip devices (SOC) scales to 45 nm and below, the side effects of substrate processing using ions may deleteriously impact device structure, properties, and fabrication.
To address these issues, an alternative to ion beam processing, atomic layer etching using neutral beams (also referred to as “fast atoms”) has been identified as a key technology proposed for future damage-free, nano-scale device fabrication. This technique enables etching of very small device structures, including etching of sub-22 nm patterns on Si wafers. For example, several experimental and theoretical studies have demonstrated damage-free and high aspect ratio anisotropic etching of sub-22 nm devices using neutral beam sources. Neutral beams by their nature are not perturbed by charge buildup on the sidewalls of features such as line structures, pillars, and trenches. Accordingly, when approaching a substrate surface, energetic neutrals in a neutral beam can travel along their initial trajectories unimpeded by effects caused by charged particles present in surface structures of a device. Damage free etching may result using a neutral beam, in contrast to standard immersed plasma reactive ion etching where a substrate is subjected to multiple fluxes of impinging species such as ions, UV photons, electrons, and neutrals.
Neutral beam processing is accordingly viewed as a promising technology due at least in part to the combination of the directionality of energetic neutrals and the lack of complications that arise in charged particle beams. Potential uses of neutral beams include spacer etch of fin-type field effect transistors (finFETs), as well as use in self-aligned double patterning lithography (SADPL). Neutral beam processing may also be employed to enable damage free material modification to facilitate patterning such as performing implantation into a spacer and performing a subsequent soft etch process.
In conventional implementation, neutral beams are created for example by directing an ion beam at a glancing angle against a grounded surface, such as a graphite material. The neutrals created are then directed toward an intended substrate. In order to achieve a high degree of etch anisotropy, a key parameter for neutral beam processing is the ability to generate a narrow neutral beam angular distribution (NAD) about a well defined mean angle to a surface of a substrate. In addition, control of the mean beam angle of incidence of an ion beam upon a neutralizer is also important to achieve a high degree of neutralization.
In known techniques for neutral beam processing, grids or circular apertures are used to produce neutral ions that may be directed to a target to be processed. However, such apparatus provide limited processing flexibility. In addition, an image of the apertures that transmit a neutral beam may be transferred to a substrate that is exposed to the neutral beam thereby leading to non-uniform treatment of the substrate.
In view of the above, it will be appreciated that there is a need to develop improved techniques and apparatus for neutral beam processing of a substrate.